1. Field of the Invention
The invention relates to an arrangement for reading out an information signal from a magnetic record carrier, comprising:
(a) a read head having a magneto-resistive element with a first terminal connected to a first point of constant potential and with a second terminal; PA1 (b) a bias-current generator for supplying a bias-current; and PA1 (c) an amplifier circuit having (i) an output terminal for supplying the information signal, (ii) a first terminal coupled to the bias-current generator and to the output terminal, (iii) a second terminal coupled to the second terminal of the magneto-resistive element so as to form a series arrangement of the bias-current generator, the amplifier circuit and the magneto-resistive element between a second point of constant potential and the first point of constant potential, (iv) a first transistor, (v) a feedback circuit having an input terminal coupled to the output terminal of the amplifier circuit and having an output terminal, (vi) a load impedance and (vii) a capacitor element, a first main flow terminal of the first transistor being connected to the second terminal of the amplifier circuit, a second main flow terminal of the first transistor being coupled to the first terminal of the amplifier circuit, a control terminal of the first transistor being coupled to the output terminal of the feedback circuit and to the first point of constant potential via at least the capacitor element, and the load impedance being coupled to the output terminal of the amplifier circuit.
2. Description of the Related Art
Such an arrangement is known from U.S. Pat. No. 5,270,882. In this known arrangement, current biasing and current sensing of the magneto-resistive (MR) element is combined, thereby providing a low-noise amplification. Read heads with MR elements and thin film inductive write heads, mounted on arms, are used in mass storage devices such as disk drives. Disk drives are equipped with multiple platters, each side of which is used for writing and reading the information. In modern systems, servo patterns are embedded in radial patterns between the data on the disk. Writing of data is interchanged with reading to remain on track. Read-to-write and write-to-read transient times should be as short as possible. The capacitor element and the feedback circuit in the known arrangement provide a low bandwidth and, therefore, the transient response time is intrinsically long.
An approach to realize short transient times is to leave the read amplifier powered during writing. This, however, results in a loss of extra power. Moreover, crosstalk from the write drivers to the read amplifiers will influence the DC settings of the read amplifiers. The DC-offset caused by the crosstalk is amplified in the read amplifier and may overload the output stages thereof. Similar problems are encountered when the information on the platters is arranged in "cylinders". During reading, the system switches from platter to platter without mechanical delay introduced by repositioning of the armstack. To avoid electrical delays, head switching should be fast. Powering a read amplifier for each read head is not attractive, in particular, in battery operated portable equipment. A second option is to use only one read amplifier and to switch the individual read heads in circuit connection with the read amplifier.
Applying this technique to the known arrangement would seriously limit the speed of switching between the different MR elements. MR elements have large spread in resistance and bias current settings. After each head change, the capacitor element in the amplifier circuit has to be charged or discharged to the new desired voltage. However, in order to obtain a sufficiently low cut-off frequency of the feedback circuit, the DC resistance of the feedback circuit should be high. A typical cut-off frequency is 10 KHz. In the known arrangement, the feedback circuit is a transconductance stage having a low transconductance Gm. The maximum output current of such a transconductance stage severely limits the speed of charging and discharging of the capacitor element and gives rise to transients with a duration of hundreds of microseconds. This is too long for fast mass storage devices. In the known arrangement, the transductance Gm of the transconductance stage is temporarily increased by increasing its bias current. This, however, causes unacceptable errors because of increased offset of the transconductance stage.